Processes for the treatment of gold/silver ores or concentrates, whose precious metal content is only partly recoverable by the traditional cyanide leaching of refractory ore, have been studied extensively. Refractory ores present particular problems in that the precious metals are trapped inside a refractory matrix. Minerals such as polybasite and tetrahedrite contain silver as well as other metals. With conventional cyanide leaching techniques, it may be uneconomical to recover the silver contained within these minerals.
Most of the known processes for use on refractory ores rely on pretreating the ore prior to cyanide leaching:
1. Roasting
This process is used on sulphide ores or concentrates and involves the heating of the material to 700.degree. C. approximately (the temperature will vary depending upon the exact nature of the feed) to oxidise the sulphur component prior to cyanide leaching. Roasting plants are capital intensive and produce as a by-product sulphur dioxide. Further costs are usually associated with the treatment or disposal of this product.
2. Pressure Leaching
The refractory material is subjected to an acid leach at high temperature (from 150.degree. to 220.degree. C.) and pressure (up to 500 psi approximately). The sulphur bearing components are oxidised and are either recycled as acid back to the leach or converted to sulphates and disposed of with the tailings following cyanidation. Capital costs are very high because of the requirements for pressure autoclaves and an oxygen plant but the process avoids problems with the disposal of the removed sulphur. However, careful disposal and management of liquid effluents is necessary.
3. Biological Leaching
Biological leaching of refractory ores is relatively new and involves the use of bacteria as the oxidising agent for the sulphur content of the ore. The reactions are performed at low temperature (from 30.degree. to 40.degree. C.) in stirred vessels of simple construction. However, reaction rates are slow and large numbers of vessels are required, which increases capital cost. The process also generates acid, again raising the question of disposal and its associated costs.
A number of chemical additives have also been used in the treatment of precious metal concentrates or ores to increase the rate of cyanide leaching or to increase the recovery of metals. These have been used during the leaching process and not as pretreatments. The capital cost associated with the use of such additives is far lower than for those options previously described. However, their use is generally restricted to specific ore types and their effect is related to improved reaction kinetics rather than improved metal recoveries.
A number of lead-based chemicals such as lead oxides and lead nitrates are used in low concentrations (50-250 ppm) primarily to enhance gold recoveries. Although the chemicals are relatively cheap ($1,500/tonne approximately) and used in small quantities, their use is restricted to specific ore types that have sulphidic components which are soluble/partly soluble in cyanide leaching. The soluble sulphides interfere with the recovery of gold, the lead additives acting to remove them from solution by means of precipitation as lead sulphate.
Recently the use of hydrogen peroxide has been proposed by Degussa AG. The hydrogen peroxide is used to boost dissolved oxygen concentrations in solution and consequently increase the kinetics of gold extraction. Hydrogen peroxide degrades very quickly and must, therefore, be dosed continuously into the leaching system. Extreme care must be taken to monitor the level of hydrogen peroxide in solution--levels too high will catalyse the degradation of cyanide and so increase operating cost. On the other hand, levels too low will have little effect on gold extraction.
Other approaches to improve precious metal recovery have been to increase leaching residence times and increase solution cyanide concentrations. The first option has obvious adverse effects on capital costs and the second on operating costs.
In many instances, poor recoveries of silver and/or gold may be accepted because the options for increasing recoveries are not economically viable.